The following relates generally to wireless communication, and more specifically to synchronization across transmitting nodes using a shared radio frequency spectrum.
Wireless communications systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems. A wireless multiple-access communications system may include a number of base stations, each simultaneously supporting communication for multiple communication devices, which may be otherwise known as user equipment (UE).
Some modes of communication may enable communication between a base station and a UE in a shared radio frequency spectrum band, or in different radio frequency spectrum bands (e.g., in a licensed radio frequency spectrum band and a shared radio frequency spectrum band) of a cellular network. However, in contrast to a carrier in a licensed radio frequency spectrum band, which may be allocated for use by the devices of one public land mobile network (PLMN) and be available to a base station or a UE of the PLMN at predetermined (or all) times, a carrier in a shared radio frequency spectrum band may be available for use by the devices of the PLMN intermittently. This intermittent availability may be a result of contention for access to the carrier of the shared radio frequency spectrum band, between devices of the PLMN, devices of one or more other PLMNs, and/or other devices (e.g., Wi-Fi devices). For some radio frames, a device of a PLMN may win contention for access to a carrier in the shared radio frequency spectrum band, while for other radio frames, the device may not win contention for access to the carrier in the shared radio frequency spectrum band.
In some contention-based systems, a base station or UE may perform a listen-before-talk (LBT) procedure to contend for access to the shared radio frequency spectrum band. An LBT procedure may include performing a clear channel assessment (CCA) procedure to determine whether a channel of the shared radio frequency spectrum band is available. When it is determined that the channel of the shared radio frequency spectrum band is available, a channel reservation signal (e.g., a channel usage beacon signal (CUBS)) may be transmitted to reserve the channel. When it is determined that the channel is not available, a CCA procedure may be performed for the channel again at a later time.
Because of the intermittent availability of carriers in a shared radio frequency spectrum band, base stations and UEs may use techniques that provide fairness in coexistence with other users of the shared radio frequency spectrum band, and that still provide reliable communications. For example, such techniques may include transmitting some information or types of radio frames using the dedicated radio frequency spectrum band and transmitting other information (e.g., lower priority information) or radio frames using the shared radio frequency spectrum band. Such techniques may be referred to as License-Assisted Access (LAA).
When different base stations of a same PLMN operator transmit over a dedicated radio frequency spectrum band in parallel, the transmission frequency used by a first base station in a first cell may, in some cases, be reused by other base stations in other cells. The rate at which a same frequency may be used in a network may be referred to as the frequency reuse rate. Some Long Term Evolution (LTE) or LTE-Advanced (LTE-A) networks may have a frequency reuse rate of one and operate in a “reuse one” mode. Frequency reuse may enhance the overall efficiency of a network by allowing additional transmissions between base stations and UEs. In systems that operate using a shared radio frequency spectrum band, enhanced frequency reuse may be desirable in order to enhance overall system efficiency.